It works a little better - it tells me when "the security code" changes. Thus an unnoticed mitm attack has to begin at the beginning of the conversation or at a

It works a little better - it tells me when "the security code" changes. Thus an unnoticed mitm attack has to begin at the beginning of the conversation or at a plausible change of the device. The attacker must be ready to intercept the "you got a new phone? Which one did you get? How broke the previous one?" and keep the conversation plausible. There are certainly targets where such attacks are relevant, but for many it's quite good.